test+fix(stammbaum): enlarge marriage-line midpoint dot to r=6 (#361)

Once the dot starts stacking to disambiguate multiple marriages on
multi-spouse rows it carries meaning, so it's no longer decorative —
WCAG 1.4.11 (3:1) applies. r=6 (12 px diameter) covers the contrast
gap; the existing brand-navy fill against the gutter and surface
backgrounds satisfies the ratio without a hue change.

Impl-ref table in stammbaum-tree-spec.html updated to match (r=6 /
12 px dia / Informational), with the WCAG reference noted.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -319,7 +319,7 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 			<circle
 				cx={(aCenter.x + bCenter.x) / 2}
 				cy={(aCenter.y + bCenter.y) / 2}
-				r="4.5"
+				r="6"
 				fill="var(--c-primary)"
 			/>
 		{/if}

frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts

@@ -315,6 +315,35 @@ describe('StammbaumTree viewBox', () => {
 		expect(slanted).toHaveLength(0);
 	});
 
+	it('renders the marriage-line midpoint dot at r=6 for WCAG 1.4.11 informational contrast (#361)', async () => {
+		// Once the dot stacks to disambiguate multiple marriages it carries
+		// meaning, so it moves from "decorative" to "informational" and the
+		// 3:1 contrast rule requires the larger 12 px diameter.
+		render(StammbaumTree, {
+			nodes: [
+				{ id: ID_A, displayName: 'Anna', familyMember: true },
+				{ id: ID_B, displayName: 'Bertha', familyMember: true }
+			],
+			edges: [
+				{
+					id: 'e1',
+					personId: ID_A,
+					relatedPersonId: ID_B,
+					personDisplayName: 'Anna',
+					relatedPersonDisplayName: 'Bertha',
+					relationType: 'SPOUSE_OF'
+				}
+			],
+			selectedId: null,
+			zoom: 1,
+			onSelect: () => {}
+		});
+
+		const dot = document.querySelector('svg circle');
+		expect(dot).not.toBeNull();
+		expect(dot!.getAttribute('r')).toBe('6');
+	});
+
 	it('centers two spouse nodes within the minimum viewBox', async () => {
 		render(StammbaumTree, {
 			nodes: [